The present invention relates to a three-dimensional fiber structure including a bent portion, a composite, and a method for manufacturing the three-dimensional fiber structure.
Fiber-reinforced composites (hereinafter, simply referred to as composites) have been widely used as lightweight structural materials. Three-dimensional fiber structures such as three-dimensional fabrics are used as reinforced base materials for composites. The three-dimensional fiber structures are required not to be a simple flat plate but have bent portions to be versatile as reinforcing materials included in composites.
Japanese Laid-Open Patent Publication No. 2-191742 proposes a technique for forming a group of warp by providing a number of warps in rows and columns in a stretched state so as to correspond to the cross-sectional shape of a three-dimensional fabric. A first weft is inserted between the columns of the group of warps, and a second weft is inserted between the rows of the group of warps. This process is repeated. The first weft and the second weft are perpendicular to the warps. As a result, the three-dimensional fabric is woven. When weaving a bent portion of the three-dimensional fabric, the first weft is partially folded back before reaching the warp of the row corresponding to the inner most layer of the bent portion. In a weaving process, the number of the rows to which the second weft is inserted is reduced in accordance with the first weft. As weaving proceeds, the woven part of the three-dimensional fabric is sequentially bent.
Japanese Laid-Open Patent Publication No. 9-137336 proposes a method for manufacturing a three-dimensional fiber structure using a frame. The shape of the frame corresponds to the shape of the three-dimensional fiber structure. The frame has a number of restricting members arranged at a predetermined pitch. The three-dimensional fiber structure includes plate-like portions and a connecting portion, which connects the plate-like portions such that the plate-like portions form a bent state. A yarn, that is, a continuous fiber is arranged folded back between the restricting members to form a laminated fiber group including fiber layers laminated on the frame. A connection yarn is inserted in the laminated fiber group retained by the frame.
However, a manufacturing apparatus used in the manufacturing method disclosed in Japanese Laid-Open Patent Publication No. 2-191742 needs to be a three-dimensional weaving machine. Therefore, the apparatus is complicated and large.
An apparatus used in the manufacturing method disclosed in Japanese Laid-Open Patent Publication No. 9-137336 is relatively simple as compared to the apparatus used in the method disclosed in Japanese Laid-Open Patent Publication No. 2-191742. However, according to the manufacturing method disclosed in Japanese Laid-Open Patent Publication No. 9-137336, a process of inserting the connection yarn into the laminated fiber group arranged on the frame in a bent state is increased. The apparatus is also complicated.
The manufacturing method disclosed in Japanese Laid-Open Patent Publication No. 9-137336, which uses the frame having the restricting members, cannot manufacture a three-dimensional fiber structure that includes bent portions that are bent in different directions. This is because the continuous fiber is arranged in a state engaged with the restricting members.
Thus, a laminated body is formed by, first, laminating fiber layers made by continuous fibers. The three-dimensional fiber structure is formed by binding the laminated body with the connection yarn. Thereafter, the three-dimensional fiber structure may be bent to finally form the three-dimensional fiber structure having a bent portion.
A three-dimensional fiber structure 61 having a hat-like cross-section shown in FIG. 9 may be manufactured by the following procedure. That is, at least biaxial flat laminated body 62 is formed by laminating fiber layers formed by continuous fibers. The laminated body 62 is bound by the connection yarn 63 to form a flat three-dimensional fiber structure. By bending the flat three-dimensional fiber structure, the three-dimensional fiber structure 61 having a hat-like cross-section is manufactured.
However, the elongation properties of the continuous fiber used in the three-dimensional fiber structure is generally very low. Therefore, the continuous fiber arranged at the outer section of the bent portion does not stretch. As a result, wrinkles 64 and distortion are generated inside the three-dimensional fiber structure 61. When the composite (FRP) includes a reinforcing material formed of the three-dimensional fiber structure 61 and a resin matrix, the wrinkles 64 and distortion cause degradation of the physical properties of the composite such as degradation of strength. The wrinkles 64 and distortion degrade the external appearance quality of the composite.